OA11282A - Fluid loss control additives and subterranean treatment fluids containing the same. - Google Patents

Abstract

Selectively cross-linked starches are disclosed that are useful as fluid loss control additives in subterranean treatment fluids comprising starches that are cross-linked to a Brabender peak viscosity of about 800 to about 1250 Brabender units after about 40 to about 70 minutes at about 92 DEG C and provide good fluid loss control over a temperature range of from about 20 DEG C to about 160 DEG C (68 DEG F to 320 DEG F).

Description

0112 8 /

TITLE

FLUID LOSS CONTROL ADDITIVES AND SUBTERRANEANTREATMENT FLUIDS CONTAINING THE SAME 5

3ACKGR0UND OF THE INVENTION

Field of the Invention

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This invention relates to cross-linked starches thatare useful as fluid loss control additives for açrueous-based subterranean treatment fluids, such as drilling,workover and completion fluids. 15

Related Background Art

The cross-linked starches of this invention may beadvantageously used in oil field applications. 20 Particularly, the starches may be incorporated into fluids used in operations where there is contact with asubterranean formation. Drilling, workover, andcompletion fluids are examples of fluids used insubterranean formations. 25

Drilling fluids may be used for any of severalfunctions that allow evaluating or producing aréservoir (formation) for oil, gas, or water. The 01 1 2 8 : drilling fluid may be pumped into the wellbore duringthe drilling operation to cool the drill bit and toflush.out the rock particles that are sheared off bythe drill bit. A "drill-in” fluid is often used while 5 drilling the production zone.

Workover fluids may be used to perform one or more of avariety of remédiai operations on a producing oil wellwith the intention of restoring or increasing 10 production. Examples of workover operations include,but are not limited to, deepening, plugging back,pulling and resetting a liner, squeeze cementing,shooting and acidizing. 15 Completion fluids may be used to perform one or more ofa variety of oil field applications illustrated by, butnot limited to, operations such as cementing, usingspacers, perforating, gravel packing, installingcasing, underreaming, milling and a variety of 20 simulation techniques such as acidizing and the like.

Subterranean treatment fluids are used in welloperations, particularly oil well operations, forvarious purposes. The subterranean treatment fluids 25 are generally prepared at the well site by admixing aviscosifying agent and a base fluid. The viscosifyingagent thickens or viscosifies the base fluid, thereby • increasing the ability of the fluid to suspend or flushout the rock particles. The subterranean treatment 30 fluid may also advantageously contain other additivesthat are conventionally used in well treatmentoperations, as needed, based upon the spécifie siterequirements and environmental conditions. 35 A common problem associated with the use of subterranean treatment fluids is the loss of fluid intothe surrounding formation near the wellbore. Fluid 3 loss control additives are added to the subterraneantreatment fluids to limit exposure of the formation andalso oontrol leak off of the liquid components to thesurrounding subterranean formation. As a resuit, the 5 subterranean treatment fluids that are most useful inwell operations possess adéquate high water rétentioncapacity. Desirably, the subterranean treatment fluidshould retain high water rétention capacity under theoften adverse environments encountered during use. For 10 example, high température conditions are encountered indeep wells, where operating températures frequentlyexceed 250°F. Low température conditions areencountered in shallow wells or in areas of a well thatare doser to the earth's surface. High sait 15 conditions are created when brine-containing subterranean treatment fluids are used. Accordingly,the fluid loss control auditive used in subterraneantreatment fluids should preferably be stable in bothhigh température and high salinity environments. More 20 preferably, the fluid loss control additive should bestable over a range of températures and should functionin environments of either high or low salinity.

Natural starches are a well known and important class 25 of matériels useful as fluid loss control additives.Kowever, it is also well known that starches do notpossess long term stability and tend to dégradé whenmaintained at elevated températures. For example, attempératures in excess of 225°?, natural or 30 conventional starches begin to dégradé, and will failto provide adéquate fluid loss control.

Several approaches hâve been used to increase thestability of starches to provide more stable well 35 drilling fluids. For example, U.S. Patent No. 4,050,968 discloses the use of quaternary ammoniumstarch dérivatives as fluid control additives that are 4 Ο 1 i 2. 8 ; stable at high températures. These dérivatives wereprepared by reaction of starch with epichlorohydrin anda tertiary amine. 5 A thixotropic three-component well drilling fluid,consisting of a cross-linked potato starch, aheteropolysaccharide derived from a carbohydrate bybact'eria of the genus Xanthomonas, and hydroxyethylcellulose, providing improved water loss ccntrol is 10 disclosed in U.S. Patent No. 4,422,947. U.S. Patent No. 4,652,384 discloses the use cf selectedcross-linked starches to provide fluid loss control atelevated températures. The starch, which is cross - 15 linked to a rather high degree under specified conditions, requires activation at elevated températurefor over four hours in order to achieve suitableeffectiveness. 20 Other well treating fluid blends hâve been prepared byincorporating Xanthomonas gum and an epichlorohydrincross-linked hydroxypropyl starch, as described in ü.S.Patent No. 4,822,500. This particular combination ofadditives interact synergistically to enhance 25 suspension characteristics and decrease fluid loss. U.S. Patent No. 5,009,267 discloses fluid loss controladditives for fracturing fluids composed of blends oftwo or more modified, or cross-linked, starches or 30 blends of one or more natural starches with one or moremodified starches.

Although many of the cross-linked starch compositionsdescribed above offer improvements over conver.tional 35 starch, there remains a need in the industry for areadily dispersible starch additive that can provide 5 0 1 ! / 6 < good fluid loss control over a wide température rangeand thac is stable in brine-containing fluids.

SUMMARY OF THE INVENTION

This invention is directed to selectively cross-linkedstarches and blends of these cross-linked starches thatare useful as fluid loss control additives that providegood fluid loss control over a wide température range.More particularly, this invention is directed to fluidloss control additives for use in subterraneantreatment fluids comprising starches which are cross -linked and hâve a Brabender peak viscosity of about 800to about 1250 Brabender units after about 40 to about70 minutes at about 92°C and provides good fluid losscontrol over a wide température range of frcm about20°C to about 160°C (o8°F to 320°F) . This invention isalso directed to the selectively cross-linked starchesthat are spray-dried to further improve the starchproperties. Additionally, this invention coverssubterranean treatment fluids containing the definedcross-linked starches.

DETAILED DESCRIPTION OF THE INVENTION

In this invention, the ability to provide a fluid losscontrol additive which is effective over a widetempérature range by using a selectively cross-linkedstarch is demonstrated. This resuit is surprising andunexpected as evidenced by a review of the literatureand commercially available products which show the useof various starches and modified starches, none ofwhich suggest the particular starches of this inventionor the degree of fluid loss control exhibited over anextended température range. 6

An important feature of this invention is the amount ofcross-linking that the starch reçoives, i.e. the amountof treatment or the âegree of cross-linking. While itis difficult to measure this characteristic of the 5 treated starch, particularly at low levers, one of thebest ways to détermine the amount of cross-linking isto measure the viscosity of the starch. It is wellknown to measure the viscosity of cross-linked starchusing a C. W. Brabender Visco-Amylo Graph. Using this 10 measuring device and method, the starches of this invention are cross-linked to provide a Brabender peakviscosity of about 800 to about 1250, preferably about920 to about 1150 Brabender units after about 40 toabout 70 minutes at about 92 °C. The test procedure for 15 measuring this feature is provided below.

The cross-linked starches used in this invention mayinclude starch treated with a number of multi-functional cross-linking agents. More particularly, 20 the cross-linking agents used in this invention includeepichlorohydrin, phosphorus oxychloride, adipic-aceticanhydrides and sodium trimetaphosphate. Epichlorohydrinand phosphorus oxychloride are preferred cross-linking·agents and epichlorohydrin is most preferred. 25

The starches which may be used as the base material inpreparing the cross-linked starch of this invention maybe derived from any plant source including corn,potato, wheat, rice, sago, tapioca, waxy maize, waxy 30 rice, and sorghum. Also useful are the conversionProducts derived from any of the above base materialsincluding, oxidized starches, prepared by treatmentwith oxidants such as sodium hypochlorite, and fluidityor thin-boiling starches, prepared by enzyme 3 5 conversions, or mild acid hydrolysis. Preferredstarches are corn, waxy maize, potato, wheat andtapioca, with waxy maize being especially preferred. 7 Ο 1 1 2 ί) ;

The cross-linked starches of the présent invention aregenerally prepared using known techniques by reactingstarch with an appropriate cross -linking agent inaqueous solution under alkaline conditions. The desired 5 cross-linked starches will hâve a specified relativelvlow degrse of cross -linking defined by Brabenderviscosity as described earlier. The amount of cross -linking agent used to achieve this degree of cross-linking will vary somewhat deper.ding of the conditions 10 and materials used. Typically, the amount of cross -linking agent used is from about 0.05% to 0.15%, andpreferably about 0.1%, by weight of the starch.

In addition to using the selectively cross-linked 15 starches as defined herein, it has been found thatpregelatinizing the starches using a spray-dryingprocess provides a product which has enhancedproperties. It is believed that the spray-driedstarches possess more uniform particle size which leads 20 to more uniform and controlled swelling. The use ofthe spray-dry pregelatinization methodology producesstarch that possesses uniform particle size without theoften significant dégradation that occurs when dryingand gelatinizing by drum-drying cr extrusion methods. 25

Pregelatinization of the cross-linked starches of thisinvention may be accomplished by spray-drying using asteam-injection/dual- or single-atomization processdescribed in U.S. Patent No. 4,280,851, U.S. Patent No. 30 4,600,472, or U.S. Patent No. 5,149,799, the disclosures of which are incorporated by referenceherein. In this process, a mixture of the granularstarch is cooked or gelatinized in an atomized State.The starch which is to be cooked is injected through an 35 atomization aperture in the nczzle assembly into thespray of atomized steam so as to heat the starch to atempérature effective to gelatinize the starch. An 8 01128/ enclosed chamber surrounds the atomization and heacingmedium injection apertures and defines a vent aperturepositioned to enable the heated spray of starch to exitthe chamber. The arrangement is such that the lapsed 5 time between passage of the spray of starch through thechamber, i.e. from the atomization chamber and throughthe vent aperture, defines the gelatinization time ofthe starch. The resulting spray-dried pregelatinizedstarch comprises uniformly gelatinized starch in the 10 form of indented spheres, with a majority of the granules being whole and unbroken and which swell uponrehydration. Nozzles suitable for use in thepréparation of these starches are described in U.S.Patent No. 4,610,760 which is incorporated by reference 15 herein. 20 25

The steam injection/dual atomization process asreferred to above may be more particularly described aspregelatinization of the starch by: a) mixing the starch in an aqueous solvent, b) atomizing the mixture with an enclosedchamber, and c) interjecting a heating medium into theatomized mixture in the enclosed chamber tocook the starch, the size and shape of thechamber being effective to maintain thetempérature and moisture control of thestarch for a period of time sufficient tocook said starch. 30 9 5 10 15 A steam injection/single atomization process forcooking and spray-drying starch is disclosed in theU.S. Patent No. 5,149,799 patent referred to above andcomprises : a) slurrying the starch in an aqueous medium, b) feeding a stream of the starch slurry at apressure from about 50 to about 250 psig intoan atomizing chamber within a spray nozzle, c) injecting a heating medium into the atomizingchamber at a pressure from about 50 to about250 psig, d) simultaneously cooking and atomizing thestarch slurry as the heating medium forces'the starch through a vent at the bottom ofthe chamber, and e) drying the atomized starch.

It is further noted that blends of the selected cross -linked starches may be used. For exemple, a blend of 20 epichlorohydrin cross-linked starch and phosphoresoxychloride cross-linked starch may be used. Theproportions of the two cross-linked starches are.notlimited but generally a weight ratio of about 4:1 toabout 1:4 of epichlorohydrin cross-linked starch to 25 phosphorous oxychloride cross-linked starch is used.

Preferably, the blend comprises a mixture of about 1:1,by weight, of the starches. The blends of theepichlorohydrin and phosphorus oxychloride cross-linkedstarches may be prepared by dry-mixing the separately 30 prepared, spray-dried starches. Alternatively, the blends may be prepared by simultaneously spray-drying wet mixtures of the cross-linked starches.

The cross-linked starches of the présent invention are35 employed in subterranean treatment fluids in an effective amount to provide fluid loss control andreduce fluid loss over a broad température range. The 10 01128, effective amount of cross-linked scarches will varydepending on the other components of the subterraneantreatment fluid, as well as the geological characteristics and conditions of the subterranean 5 formation in which it is employed. Typically, the cross-linked starch fluid loss control additive may beused in an amount of from about 1 pound to about 10pounds (lbs) of starch per barrel (bbl) of thesubterranean treatment fluid, preferably from about 3 10 to about 6 pounds per barrel. The term "barrel" asused herein means a barrel that contains 42 U.S.gallons of fluid.

In addition to the cross-linked starches, the 15 subterranean fluids may contain other components suchas a base fluid and often a viscosifying agent. Thebase fluid may be an aqueous System containing fresh water, seawater and/or brine. Brine is an aqueous r saline solution containing soluble salts of potassium, 20 sodium, calcium, zinc, and/or césium and the like. Theviscosifying agent may be xanthan gum, guar gum, otherpolymers and/or clays such as bentonite and/or mixturesof these and like materials. Other additives known tobe used in these subterranean fluids include, but are 25 not limited to, corrosion inhibitors, oxygenscavengers, antioxidants, biocides, breakers,surfactants as well as mixtures thereof and the like.

The oxygen scavengers and antioxidants may be added to 30 subterranean treatment fluids to reduce the deleteriouseffects of oxygen, i.e., the oxidative dégradation ofthe fluid loss control additive, viscosifying agent,and/or other additives. Exemplary oxygen scavengersinclude sodium sulfite, sodium dithionite, potassium 35 metabisulfite, and the like. Exemplary antioxidantsinclude magnésium oxide, triethanolamine (TEA), 11 0112b , tetraethylene pentamine (ΤΕΡΑ), and the like. Additionof oxvgen scavengers or antioxidants to subterraneantreatment fluids may provide fluids possessing ennancedvisccsity and fluid loss control properties, such t'nat 5 excellent fluid loss control may be maintained over abroad range of températures.

The amounts or proportions of each of the componentsand additives used in the subterranean treatment fluid 10 will vary greatly depending on the intended use andpurpose of the treatment fluid as well as thegeological characteristi.es and conditions of thesubterranean formation in which the fluid is employed.However, the amount cf base fluid generally présent in 15 the fluid is about 25% to about 99% by weight of thefluid. The viscosifying agent may be présent in anamount of about 0% to about 20% by weight of the fluid.Other additives, such as those listed above, may beprésent in a treatment fluid generally in an amount of 20 about 0% to about 10% by weight of the fluid.

Subterranean treatment fluids for spécifie purposesrequire spécial additives. For instance, drillingfluids may also hâve weighting agents, such as barite, 25 to control the pressure of the formation. Further information on the composition of drilling fluids canbe found in the Fifth Edition (1988) of "Compositionand Properties of Drilling and Completion Fluids" byDarley and Gray, the disclosure of which is 30 incorporated by reference herein. Oil well cernent slurries may also be classified as subterranean fluidsand often contain Portland cernent, retarders,accelerators and similar products. Weighting agents indrilling fluids and cementing agents in slurries or 3 5 spacer fluids may be used in amounts up to about 50% ormore, by weight of the fluid, depending on therequirements of the geological formation. Further 12 Ο Π 2 8 information on. the composition of cernent slurries canbe found in the 1987 SPE Monograph on "Cementing" byD.K. Smith, the disclosure of which is incorporated byreference herein. Acidizing fluids would include acid, 5 typically in amounts of about 1% to about 37% by weight, to etch the formation. The 1979 SPE Monograph"Acidizing Fundamentals" by Williams et al., thedisclosure of which is incorporated by referenceherein, further describes the uses and composition of 10 acidizing fluids. Similarly, other spécial purposeadditives could be used for other applications.

The subterranean treatment fluids of this inventionccntain the cross-linked starch or starch blend, and 15 any viscosifying agent, base fluid and other additivecomponents, présent in such proportions that areappropriate for the spécifie well site as determined bythose skilled in the art. For example, a typicaldrilling fluid containing the fluid loss control 20 additives of the présent invention may be prepared byadmixing 4 pounds of the cross-linked starch of thisinvention, 0.8 pounds of high viscosity polyanioniccellulose, 1.1 pounds of xanthan gum and 50 pounds ofcalcium carbonate into 1 barrel (42 ü.S. gallons) of 25 water or brine.

As described above, the cross-linked starch fluid lossadditives of this invention provide good fluid losscontrol over a broad température range and in an 30 environment where salinity, shear and high températuretolérance are often required. While the degree offluid loss is a relative term depending on actualconditions of operation, a fluid loss of less thanabout 100 g, as shown by the low température-low 35 pressure (LTLP) API and the high température highpressure (HTHP) API tests as described below, hasresulted when using the cross-linked starch additives 13 0112 8 of this invention. This level of fluid loss controlhas been found to occur over a broad température rangeof about 20°C to about 15O°C (68°F to 302°F) in themoderate to high salinity environment of sea water orsaturated sodium chloride solution, used as basefluids. Addition of oxygen scavengers or anticxidantsto subterranean treatment fluids containing the cross-linked starches of this invention may provide enhancedfluid loss control over a wider température range, e.g.up to about 160°C (320°F). Use of higher concentrations of fluid loss control additive and/orviscosifying agent in the subterranean treatment fluidsof this invention may similarly increase fluid losscontrol at very high températures.

The examples which follow are intended as anillustration of certain preferred embodiments of theinvention, and no limitation of the invention isimplied. In these examples, the concentration ofreagents and composition components are expressed asparts by weight, unless otherwise provided. Ailtempératures are reported in degrees Celsius unlessotherwise noted.

The following test procedures were used in evaluatingcross-linked starch fluid loss control additives inaccordance with this invention.

Brabender Viscometer Test A Brabender Visco-Amylo Graph is used in thisprocedure. This is a standard device, readilyavailable on the open market, and is a recording,rotating cup torsion viscometer that measures andrecords apparent viscosity at fixed températures ortempérature varied at a uniform rate. 14 0112 8 λ

The procedure for evaluating the cross-linked starch isas follows: 1) A sample of the cross-linked starch, prior topregelatinization via spray-drying, isslurried into a solution containing distilledwater and glacial acetic acid (2.06% byweight of total charge) to 6.0% anhydroussolids content by total weight, 2) The sample is transferred to the Brabendercup. The cup is then inserted into theviscometer, 3) The glass/mercury thermoregulator is set at92 °C and the sample is heated at a rate of .'four degrees per minute to 92 °C. The sampleis then held at about 92°C until the samplereaches the peak viscosity, and 4) The peak viscosity is recorded. Alsorecorded is the time, in minutes, that ittakes for the sample to reach peak viscosityafter it reaches 92°C (that is, the totaltime the sample is at 92 °C until the samplereaches peak viscosity).

FLUID LOSS TESTING PROCEDURE

Fluid Préparation

The starch fluid loss control additives were tested intwo aqueous Systems: seawater and 26% (w/w; saturated)NaCl brine. The seawater was prepared by dissolving18.83 g of dry “Sea-Salt” (ASTM D-1141-52, LakeProducts Company, Maryland Heights, Missouri) into 450g prepared tap water (the prepared tap water isdeionized vater containing 1000 ppm NaCl and 110 ppmCaClj) . The 2 6% NaCl base fluid was prepared bydissolving 141.4 g of NaCl into 398.6 g of deionizedwater. 15 01 1 2 8

Prier to sait addition, prepared tap water or deionizedwater was added to Kamilton Beach malt mixing cups andmixed .-at approximately 4000 rpm with a Hamilton Beachmalt mixer. A l.l lb/bbl amount of xanthan gum (1.43 gXCD, a product of NutraSweet Kelco Co., a unit ofMonsanto Company, St. Louis. Missouri) was added intoeach mixing cup and allowed to mix for approximately 3-5 minutes. One drop of 5 M potassium hydroxide wasadded to each mixing cup to raise the pH to between8.5-9 and the mixture mixed for 20 minutes at 11,000 ±200 rpm. At the end of the 20 minutes of mixing, theappropriate amount of either “Sea-Salt” or NaCl wasadded and the fluid was mixed an additional 10 minutesat 11,000 rpm. A 0.8 lb/bbl amount of AquaPAC® - Regular, which is ahigh viscosity polyanionic cellulose used as aviscositv and filtration control aid (1.07 g; a productof Aqualon Co., Houston, Texas) and a 4 lb/bbl starchsample (5.14 g), prepared as described below, were dryblended together with a spatula, then added to thefluid mixture. Mixing was continued at 11,000 rpm for15 minutes. The mixing container was removed from themixer and 50 lb/bbl CC-103 (64.29 g, calcium carbonate,a product of the ECC International Co., Sylacauga,Alabama) was added. The mixing cup was returned to themixer and mixed for an additional 5 minutes at 11,000rpm. Octanol (two drops, defoamer) was added and theresulting mixture was mixed for an additional minute.Finally, the pH of the fluid was adjusted with 5 Mpotassium hydroxide to obtain a pH between 8.5 and 9. 16 0112b <

Low Temperature/Low Pressure (LTLP) API Fluid Loss TestProcedure

Un-aged samples of the fluid prepared above were testedfor fluid loss using a standard American PetroleumInstitute (API) low température-low pressure (LTLP)Fluid Loss Test at room température (72°F).

Samples of test fluid (300 ml.) were re-mixed using aHamilton Beach Mixer for approximately 1 minute at11,000 rpm, then poured into an API Fluid Loss filtercell (Fann Instrument Company, Houston, Texas, Model12B, No. 30501) to about a half-inch from the top ofthe cell. An 0-ring and Wattman 50 filter paper wereplaced in the cell prior to sealing the cell.

The API LTLP Fluid Loss Test was performed at roomtempérature as follows. The cell was placed on afilter press, pre-set at 100 psi using nitrogenpressure, and pressurized for 30 minutes. Fluid lostfrom the pressurized cell was collected in a taredbeaker and weighed.

High Temperature/High Pressure (HTHP) Fluid Loss TestProcedure

Prior to conducting the HTHP API fluid loss test, thesamples were aged for 16 hours at elevatedtempératures, as follows.

Heat Rolling Procedure

The fluid containing the test starch sample was pouredinto a 260 ml. high-température aging cell (FannInstrument Co., Houston, Texas, Part No. 76000). Thecell is made of stainless Steel. The fluid filled thecell to approximately one-quarter inch from the top of 17 01128. the cell. The cell was capped and the outlet cap wasscrewed on. The cell was pressurized to about 150-200psi and then the valve stem was carefully tightened.

The cell was then placed in the roller oven (FannInstrument Co., Houston, Texas, Part No. 7000) that hadbeen preheated to the test température. The rolleroven is a standard API roller oven except thatEurotherm température controllers (Eurotherm Corp.,Reston, Virginia, Model 808) were added to reduce thetempérature variance during aging. The cell was rolledat the test température for 18 hours (overnight). Thesample was removed from the oven, cooled to roomtempérature, depressurized, then tested for hightemperature/high pressure (HTKP) fluid loss asdescribed below.

High Température/High Pressure (HTEP) API Fluid LossTest

The cooled sample was placed in a cool 175 ml KTHPfluid loss cell (Fann Instrument Co., Part No. 38750)containing a Wattman 50 (or équivalent) filter paper.

The bottom valve stem of the cell was closed to preventloss of the fluid prior to heat up. The top cap wasattached and the cell placed in a preheated cellholder. A nitrogen pressure line was attached to thetop valve stem and the cell was pressurized toapproximately 200 psi to prevent boiling of the fluidduring heat up. Once the cell reached température, acondenser was added to the bottom valve stem of thecell and a back pressure of 100 psi nitrogen pressurewas added to the condenser. The bottom valve stem ofthe cell was then opened to allow fluid loss to occurand the pressure of the top valve stem was increased to600 psi (to provide 500 psi differential pressure).Fluid loss was measured over a 30 minute time period oruntil complété fluid loss occurred, whichever cornes 18 Û Π 2 8 / first. The fluid loss was measured by weight. Thefluid loss reported was exactly two times the fluidloss collected (as per API procedures) to compensatefor the smaller surface area of the filter papercompared to the low température, low pressure fluidloss cell.

Différences between LTLP and HTHP Testing

Testing was conducted as per "API Recommended Practice,Standard Procedure for Field Testing Water-BasedDrilling Fluids," API RP 133-1, First Edition, June 1,1990. Room température (72°F) fluid loss tests wereconducted using the API low-temperature/low-pressure(LTLP) test procedure (API Proc. RP 13B-1 Sect. 3.3).

Ail fluid loss testing above room température was doneusing the API high-temperature/hig'n-pressure (HTHP)test procedure (API Proc. RP 13B-1 Sect. 3.5). TheHTHP testing uses different eauipment than the LTLPtest which allows for heating of the filter press andhigher differential pressures. The HTHP testing uses500 psi differential pressure whereas the LTLPapparatus uses 100 psi differential. Also, the HTHPuses filter paper that is one-half the surface area of the LTLP test and, therefore, the fluid loss reportedfor HTHP testing is doubled that collected. EXAMPLE 1

Préparation and Testing of Epichlorohydrin Cross-LinkedStarch

At room température, 1000 g of waxy maize starch wasslurried in 1500 g of water. To the slurry, sodiumhydroxide, as a 3% solution, was slowly added to a pHof about 12.0 (25 ml. of reaction slurry should reauire18-20 ml. of 0.1 N HCl to neutralize at the 19 01128^ phenolphthalein end point). Epichlorohydrin {0.13% byweight) was added to the slurry.

The reaction mixture was allowed to react at 40°C for17 hours cooled to room température, and neutralized toa pH of 6.0 with 10-30% solution of hydrochloric acid.The starch was then filtered, was'ned and dried toprovide an ungelatinized dry powder. A sample of thecross-linked starch was analyzed to détermine its peakviscosity using a C. W. Brabender Visco-Amylo Graph, asdescribed above, and found to hâve a peak viscosity of1020 Brabender units after 52 minutes at 92°C.

The dried.· cross - linked starch was slurried in water to20-30% anhydrous solids by weight. The starch wasspray-dried to pregelatinize, using the processdescribed above, and in U.S. Patent No. 4,280,851 andU.S. Patent No. 4,600,472.

The resulting dried, pregelatinized powder was testedfor fluid loss using both the API LTLP fluid loss test(room température of 72°F) and the API HTHP test,described above. The test was in both seawater andsaturated NaCl solution (26%) and gave the resultsshown below in Tables 1 and 2. 20 ΟΊ

Table 1 - Epichlorohydrin Cross-LinkedStarch/Sea Water Fluid Loss 'Température (°F) Fluid Loss (g) 72 6.4 100 6.9 150 12.5 175 18.5 225 - 46.1 250 45.7 270 57.0 290 64.8 Table 2 - Epichlorohydrin Cross-Linked

Starch/NaCl Solution1 Fluid Loss

Température (°F) Fluid Loss (g) 72 4.9 100 6.9 150 9.1 175 21.3 225 48.7 250 64.7 270 55.0 280 15.3 1 Saturated aqueous NaCl (26%) solution 21 EXAMPLE 2

Préparation and Testing of Phosphcrus OxychlorideCross-Linked Starch

At room température, 1000 g of waxy maize starch wasslurried into an aqueous solution of sodium chloride(1500 g water, 0.5%. NaCl by weight of starch).

To this slurry, a 3% solution of sodium hydroxide wasslowly added to a pH cf about 12.0 (25 ml. of reaction .slurry should require 16-18 ml. of 0.1 N HCl toneutralize at the phenolphthalein end point).

Phosphorus oxychloride (0.1%) was added and thereaction mixture allowed to react for 35 minutes. Theresulting reaction mixture was neutralized to a pH of6.0 with a 10-30% solution of hydrochloric acid. Thestarch was then filtered, washed and dried. A sampleof the cross-linked starch was analyzed to détermineits peak viscosity using a C. W. Brabender Visco-AmyloGraph and found to hâve a peak viscosity of 1000Brabender units after about 40 minutes at 92°C. Thecross-linked starch was spray-dried and tested forfluid loss as in Example 1 with the results shown below in Tables 3 and 4.

Table 3 - Phosphorus OxychlorideCross-Linked Starch/Sea Water Fluid Loss

Température (0 F)

Fluid Loss (g) 72 6.2 250 8.1 260 23.9 22 011/8/

Table 4 - Phosphorus OxychlorideCross-Linked Starch/NaCl Solution1 Fluid Loss•'Température (eF) Fluid Loss {g) 72 5.3 250 21.0 260 75.6 1 Saturated aqueous NaCl ¢26¾) solution EXAMPLE 3 A blend (1:1 wt. ratio) of epichlorohydrin (epi) crosslinked starch and phosphorus oxychloride cross-linkedstarch (both prepared as in Examples 1 and 2)was made and tested for fluid loss in sea water andsaturated NaCl solutions as in the previous Examples.The results are shown below in Tables 5 and 6.

Table 5 - Blend of Epi/Phosphorus OxychlorideCross Linked Starches (1:1) in Sea Water Fluid Loss

Température (°F) Fluid Loss (g) 72 6.9 100 6.1 150 15.7 250 33.8 290 47.9 23 011 28

Table 6 - Blend of Epi/Phosphorous OxychlorideCross-Linked Starches (1:1)in NaCl Solution1 Fluid Loss

Température (°F)

Fluid Loss (g) 5 72 100

ISO 250 1 Saturated aqueous 10 s.o S.3 9.7 31.0

NaCl (26%·) solution

Other variations or modifications, whicn will beobvious to those skilled in the art, are within thescope and teachings of this invention. This invention 15 is not to be limited except as set fortîi in the.following daims.

Claims (23)

24 We Claim: 01128/

1. A fluid loss control additive for use in asubterranean treatment fluid to provide good fluidloss control over a température range of fromabout 20°C to about 16C°C comprising a cross-linked starch having a Brabender peak viscosity offrom about 800 to about 1250 3rabender units afterabout 40 to about 70 minutes at about 92°C whensubjected to a Brabender viscometer test.

2. The fluid loss control additive according toclaim 1 wherein the starch is cross-linked with anagent selected from the group consisting ofepichlorohydrin, phosphorus oxychloride, adipic-acetic anhydride and sodium trimetaphosphate.

3. The fluid loss control additive according toclaim 2 wherein the starch is selected from thegroup consisting of corn, waxy maize, potato,wheat and tapioca.

4. The fluid loss control additive according toclaim 3 wherein the cross-linking agent isepichlorohydrin or phosphorus oxychloride.

5. The fluid loss control additive according toclaim 4 wherein the starch is waxy maize.

6. The fluid loss control additive according toclaim 3 wherein the cross-linked starch has aBrabender peak viscosity of from about 920 toabout 1150 Brabender units after about 40 to about70 minutes at about 92 °C. 25 Ο Π 2 8,

7. The fluid loss control additive according toclaim 6 wherein the cross-linked starch exhibits afluid loss of less than about 100 g when subjectedto a low-température-low pressure (LTLP) or hightempérature high pressure (HTHP) AmericanPetroleum Institute Fluid Loss Test over atempérature range of from about 20°C to about ' 16O°C.

8. The fluid loss control additive according toclaim 7 wherein the cross-linking agent isepichlorohydrin and the starch is waxy maize.

9. The fluid loss control additive according toclaim 3 wherein the cross-linked starch is about a4:1 to about a 1:4 by weight blend of epichlorohydrin cross-linked starch and phosphorusoxychloride cross-linked starch.

10. The fluid loss control additive according toclaim 9 wherein the cross-linked starch is about a1:1 by weight blend of epichlorohydrin cross -linked starch and phosphorus oxychloride cross -linked starch.

11. The fluid loss control additive according toclaim 10 wherein the starch in the epichlorohydrincross-linked starch and the phosphorus oxychloridecross-linked starch is waxy maize starch.

12. The fluid loss control additive according to anyof daims 1-11 wherein the cross-linked starch ispregelatinized by spray-drying. 26 0112 8

13. A subterranean treatment fluid compositionproviding good fluid loss control over atempérature range of from about 20°C to about16O°C comprising a base fluid, a viscosifyingagent and an effective amount of a fluid losscontrol additive which is a cross-linked starchhaving a Brabender peak viscosity of from aboutS00 to about 1250 Brabender units after about 40to about 70 minutes at about 92°C when subjectedto a Brabender viscometer test.

14. The subterranean treatment fluid compositionaccording to claim 13 wherein the subterraneantreatment fluid is a drilling fluid, a workoverfluid or a completion fluid. »

15. The subterranean treatment fluid compositionaccording to claim 14 wherein the starch is crosslinked with an agent selected from the groupconsisting of epichlorohydrin, phosphorusoxychloride, adipic-acetic anhydride and sodiumtrimetaphosphate.

16. The subterranean treatment fluid according toclaim 15 wherein the starch is selected from thegroup consisting of corn, waxy maize, potato,wheat and tapioca.

17. The subterranean treatment fluid according toclaim 16 wherein the cross-linking agent isepichlorohydrin or phosphorus oxychloride.

18. The subterranean treatment fluid according toclaim 17 wherein the starch is waxy maize. 27 01128,

19. The subterranean treatment fluid according toclaim 16 wherein the cross-linked starch has aBrabender peak viscosity of from about 920 toatout 1150 Brabender units after about 40 to about70 minutes at about 92°C.

20. The subterranean treatment fluid according toclaim 19 wherein the cross-linked starch exhibitsa fluid loss of less than about 100 g whensubjected to a low-température-low pressure (LTLP)or high température high pressure (HTHP) AmericanPetroleum Institute Fluid Loss Test over atempérature range of from about 20°C to about160°C.

21. The subterranean treatment fluid according toclaim 20 wherein the cross-linking agent isepichlorohydrin and the starch is waxy maize.

22. The subterranean treatment fluid according toclaim 16 wherein the cross-linked starch isprésent in an amount of from about 1 pound perbarrel to about 10 pounds per barrel ofsubterranean treatment fluid.

23. The subterranean treatment fluid according toclaim 22 wherein the cross-linked starch isprésent in an amount of from about 3 pounds perbarrel to about 6 pounds per barrel ofsubterranean treatment fluid.

24. The subterranean treatment fluid according toclaim 23 wherein the cross-linking agent isepichlorohydrin and the starch is waxy maizestarch. 28 01128 /

25. The subterranean treatment fluid according toclaim 16 wherein the cross-linked starch is abouta· 4:1 to about a 1:4 by weight blend of epichlorohydrin cross-linked starch and phosphorusoxychloride cross-linked starch.

26. The subterranean treatment fluid according toclaim 25 wherein the cross-linked starch is abouta 1:1 by weight blend of epichlorohydrin cross -linked starch and phosphorus oxychloride cross -linked starch.

27. The subterranean treatment fluid according toclaim 26 wherein the starch in the epichlorohydrincross-linked starch and the phosphorus oxychloridecross-linked starch is waxy maize starch.

23. The subterranean treatment fluid according to anyof daims 13-27 wherein the cross-linked starch ispregelatinized by spray-drying.